Effect of oxygen vacancy and highly dispersed MnOx on soot combustion in cerium manganese catalyst

Cerium manganese bimetallic catalysts have become the focus of current research because of their excellent catalytic performance for soot combustion. Two series of cerium manganese catalysts (Na-free catalysts and Na-containing catalysts) were prepared by coprecipitation method and characterized using XRD, N2 adsorption–desorption, SEM, Raman, XPS, H2-TPR, O2-TPD, Soot-TPR-MS and in situ IR. The effects of abundant oxygen vacancies and surface highly dispersed MnOx on soot catalytic combustion of cerium manganese catalysts prepared by different precipitants were analyzed. The activity test results show that the active oxygen species released by a large number of oxygen vacancies in the cerium manganese catalyst are more favorable to the soot catalytic combustion than MnOx which is highly dispersed on the surface of the catalyst and has good redox performance at low temperature. Because the catalytic effect of MnOx on the surface of Na-free catalysts is more dependent on the contact condition between the catalyst and the soot, this phenomenon can be observed more easily under the loose contact condition than under the tight contact condition. The activity cycle test results show that these two series of catalysts show good stability and repeated use will hardly cause any deactivation of the catalysts.

As can be seen from Fig. S2, the catalyst alone and catalyst + soot can release physically adsorbed H2O before 300°C. Since the catalysts had been calcined at 600°C for 3 h, the release of CO2 and CO is not observed during the heating process, so the peak value of the DTG curve (Tm) between 300 and 600°C is only caused by soot combustion. Figure S3. The elemental distributions determined by EDS analysis If NaOH or Na2CO3 was used as precipitant, the catalysts are flake and granular.

S3
Combined with the XRD results, it is speculated that the flake and granular parts may be composed of different phases. Therefore, the corresponding EDS and elemental S4 mapping of Na-containing catalysts are shown in Fig. S3. The atomic ratios of the corresponding elements are shown in Table S1. It's not consistent with the predicted results, Ce, Mn and Na are detected in both granular and flake parts, but the contents of each element are slightly different. The Na0.7Mn0.2O5 phase without Ce detected by XRD does not gather together independently. Therefore, for the Na-containing catalysts, the phase distribution is uniform at the micron/submicron level, but the morphology shows two kinds of structure: flake and granular. By comparing the elemental content of CM-Na and CM-NaC from element mapping, it is found that the Na/(Ce+Mn+Na) content of CM-NaC is higher than that of CM-Na. Combined with the results of the average pore diameter, this phenomenon indicates that more Na + has entered into the lattice for CM-NaC, resulting in the larger pore diameter of the catalyst. The Vis-Raman spectra of the catalysts were obtained on Thermo Scientific Dxr2xi Laser Raman spectrometer with an excitation wavelength of 532 nm. Data records range from 100 to 1000 cm -1 .
X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific TM K-Alpha TM+ spectrometer equipped with Al Kα radiation as the excitation source. All peaks were corrected with C 1s peak binding energy at 284.8 eV.

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Temperature-programmed reduction (H2-TPR) measurements were investigated to 50°C, after that the gas was switched to 3% O2/He and held for 60 min. Afterwards it was turned to He atmosphere and kept for 60 min, and followed by heating to 800°C at a rate of 10°C/min.
Soot temperature programmed reduction (Soot-TPR) was carried out on autochem 2920-hiden HPR20 (Micrometetric Co.). Prior to the test, the catalyst and soot were carefully ground in a mortar for 5 min, which were then placed in a quartz reactor. The sample was treated in Ar with high purity at 150°C for 60 min to remove any physically adsorbed impurities. Then, soot-TPR was carried out from 50 to 600 ℃ at a heating rate of 10°C/min in Ar with high purity and flow rate of 30 mL/min . The formation of COx was detected by mass spectrum.
The in situ IR spectra were recorded on a Nicolet iS50 spectrometer equipped with an in-situ diffuse reflection cell. Firstly, the background spectrum without samples was collected in He at room temperature. Then, the mixture of soot and S8 catalyst (tight contact) was pressed into a wafer and placed in the in-situ infrared transport battery. The samples were treated at 200°C in He (100 mL/min) for 60 min to remove weakly adsorbed species, and then cooled to room temperature. Finally, 5% O2/He was introduced (100 mL/min) and the samples were heated to 500°C at the rate of 10°C/min. The temperature was maintained at 500°C until the soot was burned out. The spectra were collected every 20 ℃.
Temperature programmed oxidation (O2-TPO) experiments of catalyst and catalyst + soot were carried out under the same test conditions. These experiments were carried out on utochem 2920-hiden HPR20 (Micrometetric Co.). The sample was purged at a mixture of 5% O2/He (30 mL/min) until the baseline was stable, and then it was heated from room temperature to 600 ℃ at a heating rate of 10 ℃ / min.
Mass spectrum was used to detect the COx, CO and H2O produced during the heating process